Composing white light from colored components in an optimum way has been a key problem of the lighting industry since the introduction of fluorescence lamps in the 1930s. Presently, the ability of white light to properly render the colors of illuminated objects is optimized by maximizing the general color rendering index, Ra, a figure of merit introduced by the International Commission of Illumination (Commission Internationale de I'Éclairage, CIE) in 1974 and updated in 1995 (CIE Publication No. 13.3, 1995). A trichromatic system with a maximized Ra composed of red (610 nm), green (540 nm) and blue (450 nm) components (W. A. Thornton, U.S. Pat. No. 4,176,294, 1979) is widely accepted in lighting technology as the white light standard.
The development of efficient LEDs radiating in the short-wavelength range of the visible spectrum has resulted in the emergence of solid-state lighting. Since LEDs employ injection electroluminescence and potentially offer radiant efficiency that exceeds the physical limits of other sources of light, solid-state lighting is a tremendous lighting technology with the promise of the highest electric power conservation and vast environmental benefits.
Composite white light from LEDs can be obtained by means of partial or complete conversion of short-wavelength radiation in phosphors, using a set of primary LED chips with narrow-band emission spectra or a complementary use of both  phosphor-conversion and colored LEDs. The phosphor-conversion approach based on UV and blue LEDs with complete or partial conversion in phosphors offers unsurpassed versatility in color control, since the peak wavelengths of the LEDs can be tailored by varying the chemical content and thickness of the active layers in the electroluminescent structures, and the peak wavelengths and the bandwidths of the phosphors can be tailored by varying the chemical content of the phosphor converters.
Using electroluminescent LEDs with different wavelengths and phosphors with different wavelengths and bandwidths allows for tailoring continuous illumination spectra similar to those of blackbody radiators or daylight illuminants, which are widely accepted as the ultimate-quality sources of white light. This requires the determination of LED and phosphor wavelengths and phosphor bandwidths providing the best possible quality of light for a given number of phosphors contained in a white light source, and the minimal number of phosphors with particular bandwidths required for attaining the ultimate quality of white light emitted by LEDs with partial or complete conversion.
The existing approach of assessing the color rendering properties of PC LEDs is based on the CIE 1995 procedure (CIE Publication No. 13.3, 1995), which traces back to halophosphate fluorescent lamp technology, and which employs the general color rendering index Ra based on eight test color samples selected from the Munsell system of colors (and possibly additional six test color samples). This number of colors (eight to fourteen) is much smaller than that resolved by human vision and is not suitable for tailoring phosphor blends in white PC LEDs that are designed to emit light with ultimate color quality. 